Radiation-sensitive emulsions employed in photography are comprised of a dispersing medium, typically gelatin, containing embedded microcrystals--known as grains--of radiation-sensitive silver halide. Seldom are emulsions other than silver bromoiodide employed in camera speed photographic elements. Illingsworth U.S. Pat. No. 3,320,069 discloses gelatino-silver bromoiodide emulsions in which the iodide preferably comprises from 1 to 10 mole percent. Silver bromoiodide emulsions do not consist of some crystals of silver bromide and others of silver iodide. Rather, all of the crystals contain both bromide and iodide. Although it is possible to introduce silver iodide up to its solubility limit in silver bromide--that is, up to about 40 mole percent iodide, depending upon the temperature of grain formation, much lower iodide concentrations are usually employed. Except for specialized applications, silver bromoiodide emulsions seldom employ more than about 20 mole percent iodide. Even very small amounts of iodide, as low as 0.05 mole percent, can be beneficial. (Except as otherwise indicated, all references to halide percentages are based on silver present in the corresponding emulsion, grain, or grain region being discussed; e.g., a grain consisting of silver bromoiodide containing 40 mole percent iodide also contains 60 mole percent bromide.)
A great variety of regular and irregular grain shapes have been observed in silver bromoiodide photographic emulsions. Frequently the grains are cubic or octahedral in shape. Grain edges may exhibit rounding due to ripening effects, and in the presence of strong ripening agents, such as ammonia, the grains may even be spherical. Rods and tabular grains in varied proportions have been frequently observed mixed in among other grain shapes, particularly where the pAg (the negative logarithm of silver ion concentration) of the emulsions has been varied during precipitation, as occurs, for example in single jet precipitation.
Tabular silver bromide grains have been extensively studied, often in macro-sizes having no photographic utility. Tabular grains are herein defined as those having two substantially parallel crystal faces, each of which is substantially larger than any other single crystal face of the grain. The term "substantially parallel" as used herein is intended to include surfaces that appear parallel on direct or indirect visual inspection at 10,000 times magnification. The aspect ratio--that is, the ratio of diameter to thickness--of tabular grains is substantially greater than 1:1. High aspect ratio tabular grain silver bromide emulsions were reported by de Cugnac and Chateau, "Evolution of the Morphology of Silver Bromide Crystals During Physical Ripening", Science et Industries Photographiques, Vol. 33, No. 2 (1962), pp. 121-125.
Although tabular grain silver bromoiodide emulsions are known in the art, none exhibit a high average aspect ratio. A discussion of tabular silver bromoiodide grains appear in Duffin, Photographic Emulsion Chemistry, Focal Press, 1966, pp. 66-72, and Trivelli and Smith, "The Effect of Silver Iodide Upon the Structure of Bromo-Iodide Precipitation Series", The Photographic Journal, Vol. LXXX, July 1940, pp. 285-288. Trivelli and Smith observed a pronounced reduction in both grain size and aspect ratio with the introduction of iodide.
Bogg, Lewis, and Maternaghan have recently published procedures for preparing emulsions in which a major proportion of the silver halide is present in the form of tabular grains. Bogg U.S. Pat. No. 4,063,951 teaches forming silver halide crystals of tabular habit bounced by {100} cubic faces and having an aspect ratio (based on edge length) of from 1.5 to 7:1. The tabular grains exhibit square and rectangular major surfaces characteristic of {100} crystal faces. Lewis U.S. Pat. No. 4,067,739 teaches the preparation of silver halide emulsions wherein most of the crystals are of the twinned octahedral type by forming seed crystals, causing the seed crystals to increase in size by Ostwald ripening in the presence of a silver halide solvent, and completing grain growth without renucleation or Ostwald ripening while controlling pBr (the negative logarithm of bromide ion concentration). Maternaghan U.S. Pat. Nos. 4,150,994, 4,184,877, and 4,184,878, U.K. Pat. No. 1,570,581, and German OLS publication Nos. 2,905,655 and 2,921,077 teach the formation of silver halide grains of flat twinned octahedral configuration by employing seed crystals the halide of which is at least 90 mole percent iodide. Lewis and Maternaghan report increased covering power. Maternaghan states that the emulsions are useful in camera films, both black-and-white and color. Bogg specifically reports an upper limit on aspect ratios to 7:1, and, from the very low aspect ratios obtained by the examples, the 7:1 aspect ratio appears unrealistically high. It appears from repeating examples and viewing the photomicrographs published that the aspect ratios realized by Lewis and Maternaghan were also less than 7:1. Japanese patent Kokai No. 142,329, published Nov. 6, 1980, appears to be essentially cumulative with Maternaghan, but is not restricted to the use of silver iodide seed grains.
Wilgus and Haefner in U.S. Ser. No. 429,420, filed concurrently herewith and commonly assigned, titled High Aspect Ratio Silver Bromoiodide Emulsions and Processes for Their Preparation which is a continuation-in-part of U.S. Ser. No. 320,905, filed Nov. 12, 1981, now abandoned, were the first to prepare high aspect ratio tubular grain silver bromoiodide emulsions. The present invention represents an alternative process of preparing such emulsions. According to the process of Wilgus and Haefner the pBr (the negative logarithm of bromide ion concentration) of the dispersing medium within the reaction vessel is adjusted to a level of from 0.6 to 1.6 with the reaction vessel being initially substantially free of iodide salts. To form high aspect ratio tabular silver bromoiodide grains silver, bromide, and iodide salts are concurrently added to the reaction vessel while maintaining the pBr of the reaction vessel above 0.6.
Kofron et al U.S. Ser. No. 429,407, filed concurrently herewith and commonly assigned, titled Sensitized High Aspect Ratio Silver Halide Emulsions and Photographic Elements, which is a continuation-in-part of U.S. Ser. No. 320,904, filed Nov. 12, 1981, now abandoned, more fully discussed below, discloses chemically and spectrally sensitized high aspect ratio tabular grain silver halide emulsions and photographic elements incorporating these emulsions.